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          InnoDB的MVCC
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        <h1 id="Innodb-的-MVCC"><a href="#Innodb-的-MVCC" class="headerlink" title="Innodb 的 MVCC"></a>Innodb 的 MVCC</h1><p>说到 MVCC 大家都知道，这个是为了提高数据的并发能力。如果数据库只能有一个线程来处理数据的读取和更新，那么肯定是不行的。</p>
<p>为了解决顺序读取，那么第一步想到的肯定是，读写锁。将 Read Lock 和 Write Lock 分开，那样不就可以实现多线程操作了。</p>
<blockquote>
<p>Read Lock 和 Write Lock</p>
</blockquote>
<a id="more"></a>

<p>但是，程序员对于性能的要求是无止境的。为了能更加的提高数据库的性能，在一定的范围允许 Read 和 Write 一起执行。将数据备份多个版本，并且标记上版本号，过滤哪些程序能看到，哪些不能被看到，这就是 MVCC 所带来的价值。</p>
<p>MVCC 只是一套实现方式的概念，就像我们使用 锁一定，真正用到的时候，还需要区分我们是乐观锁还是悲观锁。不同的业务场景，所需要的隔离情况不同。</p>
<p>标准的SQL规范规定了4个界别：</p>
<table>
<thead>
<tr>
<th></th>
<th>中文名</th>
<th>备注</th>
</tr>
</thead>
<tbody><tr>
<td>Serializable</td>
<td>串性操作</td>
<td></td>
</tr>
<tr>
<td>Repeatable Read</td>
<td>可重复读</td>
<td>指在一个事务中，所有的读的结果都是一样</td>
</tr>
<tr>
<td>Read Committed</td>
<td>读已提交</td>
<td>在一个事务中，会读到已提交的事务，多次读的结果会不一样</td>
</tr>
<tr>
<td>Read Uncommitted</td>
<td>读未提交</td>
<td>在一个事务中，会读到未提交的事务</td>
</tr>
</tbody></table>
<p>在不同的事务隔离级别之下，MCVV 带来的效果也是不一样的。</p>
<h2 id="实现的基础"><a href="#实现的基础" class="headerlink" title="实现的基础"></a>实现的基础</h2><p>为了达到，不同事务看到的数据是不同的，那么第一点就是对于已有的数据，增加数据结构。</p>
<table>
<thead>
<tr>
<th>上一个事务Id</th>
<th>指向 undo.log</th>
<th>PK</th>
<th>数据内容</th>
</tr>
</thead>
<tbody><tr>
<td>DB_TRX_ID</td>
<td>DB_ROLL_PTR</td>
<td>DB_ROW_ID</td>
<td>data</td>
</tr>
</tbody></table>
<blockquote>
<p><a href="https://dev.mysql.com/doc/refman/8.0/en/innodb-multi-versioning.html">https://dev.mysql.com/doc/refman/8.0/en/innodb-multi-versioning.html</a><br>顺便一说，mysql的官方文档写的挺好的，还是需要看官方文档。</p>
</blockquote>
<p>DB_TRX_ID指向的是上一个事务的Id, 同时这是事务Id是递增的。如果实在一个分布式系统中，可以使用时间戳，来保证递增性。</p>
<p>DB_ROLL_PTR 指向的是 undo.log 。在一次事务中，数据可能被更新多次，row 的值需要根据 DB_ROLL_PTR 链表往上找。</p>
<h2 id="undo-log-什么时候会被删除"><a href="#undo-log-什么时候会被删除" class="headerlink" title="undo.log 什么时候会被删除"></a>undo.log 什么时候会被删除</h2><blockquote>
<p>Update undo logs are used also in consistent reads, but they can be discarded only after there is no transaction present for which InnoDB has assigned a snapshot that in a consistent read could need the information in the update undo log to build an earlier version of a database row.</p>
</blockquote>
<blockquote>
<p>update的undo log也在一致性读中被使用。但是他的删除和insert的unlog的不一样。在一个事务中为了保证一致性读，InnoDb 会使用 undo log来构建一个早些版本的 mysql 行数据。只有当I nnoDB 没有相关的 snapshot 的时候，undo log会被删除。</p>
</blockquote>
<h2 id="MVCC-对于-Secondary-Indexes-和-clustered-index-不同处理"><a href="#MVCC-对于-Secondary-Indexes-和-clustered-index-不同处理" class="headerlink" title="MVCC 对于 Secondary Indexes 和 clustered index 不同处理"></a>MVCC 对于 Secondary Indexes 和 clustered index 不同处理</h2><p>由于 clustered index 挂载的是真正的数据，所以数据会被立刻更新。而 Secondary Indexes 则是通过标记旧的数据失效，同时插入一条真的索引，来实现。</p>
<p>查询Secondary Index时，如果使用了事务，Mysql 会去搜索 cluster Index 挂载的 row，通过DB_ROLL_PTR 指向的 undo.log 来决定是否可见。</p>
<h2 id="Readview"><a href="#Readview" class="headerlink" title="Readview"></a>Readview</h2><p>用来控制不同的事务是否可见，mysql维护了一份全局事务链表。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">ReadView&#123;</span><br><span class="line">    int maxTrxId;</span><br><span class="line">    int minTrxId;</span><br><span class="line">    int TrxId;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>ReadView记录了当时所有未提交的事务。RC 是在每次读的时候，生成ReadView。为什么每次都要生成一个，不能直接用全局的事务链呢？可能是先要记录一份快照，怕读数据的时候，快照发生变化。</p>
<p>而RR是在事务的开始就直接生成了ReadView。</p>
<h2 id="为什么Mysql的默认事务隔离级别是-RR"><a href="#为什么Mysql的默认事务隔离级别是-RR" class="headerlink" title="为什么Mysql的默认事务隔离级别是 RR"></a>为什么Mysql的默认事务隔离级别是 RR</h2><p>因为在Mysql 5.1 版本之前，binlog 只有 statement 模式。也就是复制 sql 语句，在主从配置下，如果slave按照 master的 sql 实现数据复制，如果是 RC 模式会出现 数据错乱。因为，默认的配置是 RR。</p>
<h2 id="灵魂拷问：RR-是否完全解决了幻读-（Phantom-Row）"><a href="#灵魂拷问：RR-是否完全解决了幻读-（Phantom-Row）" class="headerlink" title="灵魂拷问：RR 是否完全解决了幻读 （Phantom Row）"></a>灵魂拷问：RR 是否完全解决了幻读 （Phantom Row）</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TABLE</span> `test_rr` (</span><br><span class="line">  `id` <span class="type">int</span>(<span class="number">11</span>) <span class="keyword">DEFAULT</span> <span class="string">&#x27;1&#x27;</span>,</span><br><span class="line">  `vale` <span class="type">int</span>(<span class="number">11</span>) <span class="keyword">DEFAULT</span> <span class="string">&#x27;1&#x27;</span></span><br><span class="line">) ENGINE<span class="operator">=</span>InnoDB <span class="keyword">DEFAULT</span> CHARSET<span class="operator">=</span>latin1;</span><br></pre></td></tr></table></figure>
<p>事务1：</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> test_rr</span><br><span class="line"><span class="keyword">VALUES</span> (<span class="number">1</span>, <span class="number">1</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">begin</span>;</span><br><span class="line"><span class="keyword">select</span> <span class="operator">*</span> <span class="keyword">from</span> test_rr;</span><br><span class="line"></span><br><span class="line"><span class="comment">--- 这里执行事务2 </span></span><br><span class="line">## <span class="keyword">INSERT</span> <span class="keyword">INTO</span> test_rr</span><br><span class="line">## <span class="keyword">VALUES</span> (<span class="number">2</span>, <span class="number">2</span>);</span><br><span class="line"><span class="comment">---</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">UPDATE</span> test_rr <span class="keyword">SET</span> vale<span class="operator">=</span><span class="number">3</span>;   ## 这里你会发现神奇现象发生了</span><br><span class="line"><span class="keyword">select</span> <span class="operator">*</span> <span class="keyword">from</span> test_rr;       ## 在这里，你居然能看到两条数据了</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>事务2：</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">begin</span>;</span><br><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> test_rr</span><br><span class="line"><span class="keyword">VALUES</span> (<span class="number">2</span>, <span class="number">2</span>);</span><br><span class="line"><span class="keyword">commit</span>;</span><br></pre></td></tr></table></figure>
<p>神奇的结果就会产生，事务1居然搜索出了所有的数据。</p>
<p>Mysql 的读分为两种：快照读 和 当前读<br>如果使用 select ….where id &gt; 100 for update 则为当前读，另外的则为快照读。因此，如果不使用 for update 的间隙锁，来限制其他事务的 insert。 那么也就意味着，如果select 是范围查询的话，有可能会查出“新的数据”。典型的就是，之后再使用一个 update where id &gt; 100这种。</p>
<p>另外，比较神奇的是，update 还是可以成功的。讲道理不应该是回滚吗？找到的解释，也就只有下面的这个。</p>
<blockquote>
<p>snapshot isolation要求两个并发事务的写写冲突要回滚其中一个。MySQL并没有实现严格的snapshot isolation， MySQL认为回滚会影响性能，它允许并发的事务更新已提交的数据。这样，MySQL会产生一些其它MVCC数据库没有的异常</p>
</blockquote>

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